The fact that we do not know for sure who is knocking at the door gives the story more of a feeling of realism than if we found out it was the dead and horribly disfigured son returning home. The entire story remains realistic, although it is based on old tales of people granted three wishes by someone with magical powers. The monkey's paw may be nothing but an old curio with a legend attached to it. The son's death might have no connection with the wish for money. The person knocking at the door might be anybody. The intrusion of a supernatural event at the very end could spoil the overall effect of this story.
Like my colleagues, it is the not knowing which makes the ending so effective. Edgar Allan Poe was the master of this horror technique, and it still works today. Let's say the author describes what he thinks is the most horrific thing anyone can imagine; if a reader is not horrified, the effect is lost and the story fails. If, however, he leaves the horror to his readers' imaginations, the story will always be successful.
I agree that the ending of "The Monkey's Paw" does capture the reader's attention. It as if the reader is holding his or her breath, wishing that Mr. White would hurry and find that paw. The reader is left with the image of a horrible scene outside that door. A first reading definitely intrigues the reader, causing an anguish for the reader. If Herbert gets in the house, the mother will wish he hadn't. I think that the part I find most interesting is the fact that Mr. White made the second wish to please his wife. Of course, he had not really had time to think it through. The two miles that it took for Herbert to walk home gave him time to think about how gruesome his son would be on the other side of that door.
I think the ending of "The Monkey's Paw" is absolutely ingenious. I think, as I always do about horror stories or films, that it's better NOT to know or see what is truly scary because our imaginations can ALWAYS do a better job of frightening us than any movie director or author can. The best example (and one that seems really dumb, actually, when taken out of context) is the darn fire hose/extinguisher in King's novel The Shining. It was months until I could walk by one of those in a hotel without being a bit wary. (Ridiculous! Ha!) It is the short story "The Monkey's Paw" that provides yet another perfect example. The reader imagines the absolute worst case scenario of Herbert knocking on that door, ... possibly one of the undead, ... possibly a mummy, ... possibly a rotting corpse, ... but most definitely the absolute scariest, and yet specific image to that person, that could possibly be imagined! Great job, Jacobs, ... great job!
I agree with other editors by focusing at the way that horror fiction that does not reveal but leaves it to our imagination is much more terrifying than horror fiction that reveals. Our mind is much more able to paint a picture of terror and horror than any words can, and thus by leaving a door separating the dead son and his mother, we are left to imagine what was behind that door, and are as thankful as the father that he manages to make his third wish just in time.
The third wish of the ending follows the pattern of the characters' actions in The Monkey's Paw. For, each time that the Whites have made a wish upon the monkey's paw, they have failed to be specific and to think through the ramifications and consequences of their desires. When Mr. White wishes for the three hundred pounds, for instance, he does not stipulate how this money is to be procured, and, of course, this omission is what causes the tragedy. Again, when Mrs. White wants her boy back, she forgets to add that he be as he was before the accident.
I think it is much better that the author did not reveal who the knock on the door came from at the end of the story. Today's fright/shock movie directors probably could not resist the urge to present a gruesome, mangled Herbert waiting at the door, but the author's decision to allow the reader's imagination to rule adds a bit of mystery to the ending as well.
It makes me wonder to some extent how much of the ending is simply in the minds of the Whites. Did their son really come back from the dead? Is it really he who is banging on the door? Or is it just a coincidence that there is someone banging on the door who then stops. I am not completely sure as to whether Herbert is really raised from the dead or not.
I think a more tragic, but perhaps less suspenseful, ending would be for Mrs. White to have opened the door before Mr. White made his last wish, have Herbert be returned just fine rather than mangled, only to die in front of her as Mr. White makes the wish.